The present invention is directed to a mechanical pencil of the collet action type wherein a shiftable lead is carried within a collet which collet member is held in lead gripping relation by the effect of a compression spring and which is movable in opposition to the bias of the spring to cause the lead to be moved extensibly from the collet.
In the case of such types of mechanical pencils where the collet member is closed by the action of a spring and which are designed to be operated by means of a depressable push button opposing the bias of the spring, the lead has to be held firmly even after frequent releasing of the collet member so that with sufficient safety the individual's firm writing pressure can be absorbed without the shifting of the lead. It is desirable that the collet member clamping effect be achieved with as little prestress of the spring as is reasonably possible and with a low spring constant in order to facillitate the operation of the depressable push button.
It has heretofore been known to construct collets of metal for fulfilling these requirements and such metal collet members are generally slotted several times at their circumference and have a thread at their far end to receive the lead guiding tube whereby simultaneously a contact surface for the compression spring is formed which causes the clamping force to be exerted at the collets. In order to achieve a precise application of the clamping jaws of the collet evenly at the circumference of the lead, in most cases three slots are provided in such metal collets. In addition, the clamping jaws are generally roughened in the area of the surfaces which are provided for contact with the lead or they are rendered particularly gripping by cutting of threads having a left or right hand pitch. This type of collet member has proven quite effective in the past. However, such types of collet members have the disadvantage of requiring a considerable amount of assembly work and precise machining and are consequently relatively expensive to manufacture. While such mechanically pencils are generally of high quality and are salable in particular markets (such as for use in the making of professional drawings) these pencils are generally too high priced to be used in the general market and particularly in the student market.